Transcript Class 19 – More GPS, GLONASS and related issues

Class 20 – More GPS,
GLONASS and OPUS
2 April 2008
Topics
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Homework review/answers
Recap of GPS range solution
GPS phase measurements
GLONASS
GLONASS/GPS interoperability
Homework
• How accurate does a clock need to be to
achieve measurement precision at the
millimeter level (< 1 cm)?
• c = 299,792,458 meters per second
• c *(10e-12 seconds) = 0.003m
• What is the orbital period of SV1?
• P2/a3=4π2/GMe
• a = 26,559,122 m; Geometric gravitational
constant = GMe = 3986005*108m3/s2
• period (seconds) = sqrt((a3*4π2)/GMe)
• period (hour) = 11 hours 58 minutes
Homework continued
• How many seconds in a week?
• (#days * #hours in day * #seconds in hour *
#seconds in minute) = 7*24*60*60 = 604,800
• What is the current GPS week?
– week of 30 March to 5 April 2008 is 1473
Signal Processing on-board
Frequency to Wavelength
• We can track the phase of the signal and
accumulate the number of wavelengths (and the
fractional first phase) as a measurement.
– λ = c / f ;wavelength = speed of light divided by frequency
L1 = c/f1=19 cm
L2 = c/f2 = 24.4 cm
L5 = c/f5 = 25.5 cm
c = 299792458m/s
Frequency Combinations
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Narrow-lane = f1 + f2 ≈ 11 cm
Wide-lane = f1 – f2 ≈ 86 cm
Iono-Free ≈ f1/(f1-f2) ≈ 5 cm
Why do this?
– Iono-free effectively eliminates
this effect
– Other combinations assist
integer fixing.
Integer bias ambiguity
GNSS
• Global Navigation Satellite Systems
– NAVSTAR GPS operational
– GLONASS operational
– Galileo (not yet)
– COMPASS (from The Space Review)
• “China’s existing Beidou navigation network is a
clumsy system based on three satellites, (two
operational and one reserve) in geosynchronous
orbit, launched between 2000 and 2003.” 19 June
2006
GLONASS
GLONASS
• Global'naya Navigatsionnaya
Sputnikovaya Sistema
– Intended 21 SV with 3 on-orbit spares
– 3 orbital planes separated by 120 degrees
– orbits inclined 65 degrees
– orbit period 11h 15m
– first launch 1982; most recent 25 Dec 2007
http://www.glonass-ianc.rsa.ru
Orbital elements
Current March 2K7 Status
GLONASS
Note multiple frequencies!
GPS and GLONASS
ultra-rapid orbit file
Interoperability questions
• GLONASS uses a different geocentric
datum (PZ-90)
• GLONASS time and GPS time are not the
same.
– Leap seconds are an issue
• Hardware biases
• Use of different frequencies means more
difficulties when fixing integers.
– Some broadcast negative frequencies!
GPS only planning
Nsats – Number of satellites
PDOP – Position Dilution of Precision
See G19 that sets at 23:30 and rises again at 05:30 (6 hr period)
SKYPLOT
Dilution of Precision
• A planning measure measuring the effect
of satellite geometry wrt the satellite
constellation. Smaller values are better.
• PDOP – Position (East, North and Up)
• GDOP – Geometric (E,N,U and Time)
• VDOP – Vertical (Up)
• TDOP – Time (Time)
• DOP combined with UERE to estimate
positioning accuracy.
Short Occupation Times
VDOP
Best VDOP
GPS Baselines
OPUS Solution Extended Output
GPS Antenna Calibration Issues
Obviously, GPS antennas have different physical dimensions.
Less obviously, they track the satellites differently. Note the
different values for the same elevation angle for these two
antennas.
Absolute Antenna Calibrations
Required reading: [IGSMAIL-5189] Planned changes to IGS antenna
calibrations http://igscb.jpl.nasa.gov/mail/2005/msg00111.html
This is NOT JCAD 28. It is a tidal station near
Rainbow Bridge. It is not yet in the NGS data base.
These three G-file sections show (line “C”) the baseline
components and their standard deviations for the three ties to
CORS sites. The “D” line shows the correlations between
baseline components (DX to DY, DX to DZ and DY to DZ).